1. Field of the Invention
The present invention relates to an improved method and system for communicating across a controller area network at a higher data rate while providing power to a plurality of devices wherein the controller area network uses only three wires to provide both power and data to the devices on the network.
2. Description of the Related Art
Communication between devices enables multiple devices to operate together thereby enhancing the individual devices operating capability. For example, computers across a network can communicate with one another, even around the world, providing information and sharing resources, thereby enhancing the ability of any single computer connected to the network to accomplish tasks. Increasingly, all devices are being networked together to provide synergy and enhanced communication between the devices increasing the effectiveness of each device on the network. Many of these networks are limited to small scale networks known as controller area networks. Such networks may be limited to spaces such as within a vehicle, home, or office. Regardless of the size of the network, the effectiveness of a network in allowing the connected devices to interoperate with maximum efficiency is inherently limited by the bandwidth, or communication speed, across the network. Depending on the needs of the devices connected to the network, limited bandwidth may reduce the amount of data communicated between the devices. In fact, as the number of devices on the network increases, the bandwidth needed by the devices also increases, sometimes exponentially. An overloaded network with a small bandwidth may eventually reach a point where the devices on the network cannot operate effectively with one another.
The speed of communication across the network is generally measured in the number of bits per second, bps, transmitted across the data bus. Increasing the number of bits per second that can be transmitted across the data bus increases the amount of information that can be communicated. Early modem technology operated at only 120-300 bits per second which allowed only a small amount of real-time data to be communicated across the data link. However, newer communication networks, such as Ethernet for connecting homes feature much higher communication rates ranging from 10 million bits per second to 1 billion bits per second or more across the data bus. These advanced communication networks, however, require sophisticated hardware and software to achieve the data rates disclosed and are not suitable for simpler systems because of the increased cost and overhead.
One specific type of communication system known in the art is an HVAC (heating, ventilation, air conditioning) communication system developed by Honeywell and known by the trade name Enviracom™. The Enviracom™ system has been put forth by Honeywell as a method of communicating between HVAC products, including thermostats, furnaces, cooling units and control panels, to enhance the effectiveness of the HVAC system within a particular structure or small group of structures. The advantage to this system is that the protocol and method of communicating across the network are well-known in the art and publicly disclosed. A growing number of HVAC products incorporate the Enviracom™ communication system to increase the effectiveness of the products and allow some interoperability between products of different manufacturers.
In the prior art system, a number of HVAC devices, whether thermostats, controllers, furnaces, are connected to the network with three wires. Two of the three wires provide a limited power supply to the connected devices. In the basic, well-known implementation, the two power lines provide an alternating current power source across the power lines to the devices which can be used to power the electronics of the connected device. More importantly, the third line, a data line is used to transmit and receive data across the network. The three lines, two power and one data, can be connected between each device in a variety of different configurations, including star topology, daisy-chaining, or a combination of the two.
The network of all the data lines between the individual devices on the network forms a data bus wherein any device on the network can communicate with each and every other device connected elsewhere on the network, even if the two communicating devices are located on opposite sides of the network topology. This type of communication network structure is well-known in the art and is not limited to the Enviracom™ system nor the present improvement.
Generally speaking, the Enviracom™ communication system provides 24 volts of alternating current power across the power lines at a frequency of 60 Hz. This corresponds to the natural frequency of the electrical grid in North America eliminating the need to modulate the frequency of the power lines which is more complicated than stepping down the voltage from ˜120 Vac to 24 Vac. While primarily providing a power signal across the controller area network, the power lines also serve a secondary purpose in providing a pseudo-clock signal of approximately 120 Hz for use in reading the data line. In a 60 Hz sinusoidal waveform, such as that transmitted across the power lines, the sinusoidal signal has two zero-crossings per cycle. In the communication system, the zero-crossing can be used as a clock for regulating the data transmitted across the data bus. The 120 zero-crossings per second of the 60 Hz allows for 120 bits of data to be communicated across the data bus per second, or one bit of data communicated between each zero-crossing in the previously known implementation.
The data bus also features a sinusoidal signal that is also at 60 Hz, but can feature a lower maximum and minimum voltage depending on the physical dimensions of the network. In between each zero-crossing of the power signal, the data bus is modulated to communicate the appropriate bit across the data bus. For example, if the data line contains a high-amplitude signal then a 1 bit is being transmitted across the bus, while if the data line contains a low-amplitude or even zero-amplitude waveform within the bit container then a 0 bit is being transmitted across the bus. This simple form of communication within a controller area network is extremely effective when communicating small amounts of data across the bus and will be better explained with reference to the figures and detailed description.
Although the controller area network communication system does allow for communication across the network with only a minimal amount of wires providing both power and data to the devices on the network, the practical limitation of using 60 Hz alternating current for the power limits the effective bandwidth of the communication system to 120 bps if using the simple modulation previously described. It would be greatly advantageous to provide an improvement to the current system for increasing the bandwidth of the controller area network without requiring expensive or sophisticated hardware changes. It would be desirable if this communication system used similar forms of amplitude modulation without increasing the complexity of the system thereby allowing current hardware to implement the technique with only minor software modifications.
In view of the aforementioned shortcomings, an improved system and method for communicating across a controller area network providing a higher bandwidth than is currently taught is needed.